linux-kernel - Re: [PATCH v5] ANDROID: binder: change down_write to down

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Message-ID: <20180330100407.GB19140@kroah.com>
Date:   Fri, 30 Mar 2018 12:04:07 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Ganesh Mahendran <opensource.ganesh@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Joe Perches <joe@...ches.com>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v5] ANDROID: binder: change down_write to down_read

On Fri, Mar 30, 2018 at 10:29:21AM +0900, Minchan Kim wrote:
> Hi Ganesh,
> 
> On Fri, Mar 30, 2018 at 09:21:55AM +0800, Ganesh Mahendran wrote:
> > 2018-03-29 14:54 GMT+08:00 Minchan Kim <minchan@...nel.org>:
> > > binder_update_page_range needs down_write of mmap_sem because
> > > vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless
> > > it is set. However, when I profile binder working, it seems
> > > every binder buffers should be mapped in advance by binder_mmap.
> > > It means we could set VM_MIXEDMAP in binder_mmap time which is
> > > already hold a mmap_sem as down_write so binder_update_page_range
> > > doesn't need to hold a mmap_sem as down_write.
> > >
> > > Android suffers from mmap_sem contention so let's reduce mmap_sem
> > > down_write.
> > 
> > Hi, Minchan:
> > 
> > It seems there is performance regression of this patch.
> 
> You mean "This patch aims for solving performance regression" not "This patch
> makes performance regression"?
> 
> > 
> > Do you have some test result of android app launch time or binderThroughput?
> 
> Unfortunately, I don't have any number. The goal is to reduce the number of
> call mmap_sem as write-side lock because it makes priority inversion of threads
> easily and that's one of clear part I spot that we don't need write-side lock.

Please always run the binderThroughput tests when making binder changes
(there is a binder test suite in the CTS Android tests), as that ensures
that you are not causing performance regressions as well as just normal
bug regressions :)

thanks,

greg k-h